1. Field of the Invention
The present invention relates to an ion implanter and a controlling method therefor. More particularly, it pertains to an ion implanter used to form a dopant impurity layer in a semiconductor substrate during the a manufacturing process and to a method of controlling such an ion implanter.
2. Description of the Related Art
An ion implanting method is used as one of the methods of forming dopant layers in a semiconductor substrate. FIG. 6 is a schematic view showing a conventional ion implanter. Such an ion implanter is used to implant ions into a semiconductor substrate in the following way.
First, a dopant gas or vapor from a solid is fed to an ion source 1, where an arc discharge occurs to generate a high-density plasma. Then, a high voltage (generally, 20-80 kV) is applied to an extraction electrode (not shown) to extract ions from the ion source 1. At the same time, a predetermined energy is given to the ions to convert them into ion beams 2. The ions are deflected by a mass analyzer 3. The ions each have a predetermined electrical charge and mass determined by the magnetic flux density, energy, and the curvature radius of the mass analyzer 3. Such ions are selected from the ion beams 2, to which the predetermined energy has been given.
Then, the ion beams 2 pass through a resolving aperture 4 to improve the resolution of the ion beams, and are led to an acceleration tube 5 where the energy of the ion beams is increased to a predetermined level. A quadrupole lens 6 is adjusted so that the ion beams 2 are focused on a semiconductor substrate 7, the target, disposed in an implant chamber 10. At this phase, scanning electrodes 8 and 9 scan the ion beams 2 so that ions are implanted uniformly into the semiconductor substrate 7.
The conventional ion implanter is constructed as described above. In most cases, when ion implantation is carried out, patterns are already formed on the semiconductor substrate 7. FIG. 7 shows an example of a semiconductor substrate 7 on which patterns are already formed. As shown in FIG. 7, the semiconductor substrate 7 is of a p-type. Thick field oxide insulating films 12 are disposed selectively on the obverse surface of the semiconductor substrate 7. Thin insulating films 13, acting as gate insulating films, are formed in active regions sandwiched by the field oxide insulating films 12. Gate electrodes 14 are disposed on the insulating films 13. As semiconductor devices become smaller and smaller, insulating films 13 become thinner and thinner.
When a CMOS transistor is formed, as shown in FIG. 7, a photoresist 15 usually masks a p-channel region 11. In such a case, ion implantation is performed to make the source and drain of an n-channel region N type. At this time, ions (positive ions) made of a dopant element, such as phosphorus or arsenic, are used to form ion beams 12.
When the ions are implanted in the semiconductor substrate 7 coated with the insulating films 12 and 13, and the photoresist 15, the obverse surface of the semiconductor substrate 7 becomes positively charged. As a result, there is great possibility that a dielectric breakdown will occur in the extremely thin insulating films 13.
A charge neutralizer 16 is generally used as a means of preventing such a dielectric breakdown. FIG. 8 is a view illustrating the operation of the charge neutralizer 16. As shown in FIG. 8, the charge neutralizer 16 accelerates primary electrons 17 emitted from a filament 16b of an electron gun 16a. This acceleration takes place in an electric field of approximately 300 V. The emitted primary electrons 17 then irradiate a Faraday cage opposite to the charge neutralizer 16 in order to generate secondary electrons 19. The secondary electrons 19 are supplied to the semiconductor substrate 7 into which ions are being implanted. The semiconductor substrate 7 which has been electrically charged by the positive ions is thereby neutralized electrically. The semiconductor substrate 7 is secured by a clamping ring 20 to a supporting bed 21.
When such a charge neutralizer 16 is used in ion implantation, portions of the primary electrons 17 recoil and reach the semiconductor substrate 7 while they possess an energy of approximately 300 eV. The recoil electrons exceed the positive potential of the semiconductor substrate 7, which has become positively charged with the ion beam 2, and reach an electrically neutral portion of the semiconductor substrate 7, which in turn becomes negatively charged. In other words, the ion beams 2 move on the semiconductor substrate 7 so as to scan it. For this reason, when a charge is observed at an extremely small region of the semiconductor substrate 7, it is found that the substrate 7 is positively and negatively repeatedly.
In this way, when the charge neutralizer 16 is used in ion implantation, the semiconductor substrate 7 is positively and negatively charged repeatedly, and will not be neutralized. As a result, there is a great possibility that a dielectric breakdown will occur in the extremely thin insulating film 13. Such a phenomenon becomes pronounced, particularly when the insulating film 13 has a large area. For instance, a MOS capacitor has a great possibility of a dielectric breakdown since the insulating film 13 must have a large area to increase its electric capacitance. In addition, the quantity of recoil electrons increases as the Faraday cage 18 becomes dirty. For example, when an impurity-doped insulating film is formed on the Faraday cage 18, the surface of the cage 18 becomes charged and the charge changes with time. Ion implantation cannot be controlled effectively.
When the ion implanter mentioned above is used to perform ion implantation, electrical charging occurs and causes a dielectric breakdown in smaller and smaller semiconductor devices. Consequently, such a breakdown decreases the yield as well as the reliability of the semiconductor devices. Even when the charge neutralizer 16 is used, primary electrons having high energy negatively charge the semiconductor substrate 7. Thus, the charge neutralizer 16 does not completely solve the problem of the conventional art. Moreover, when ion implantation is carried out so as not to cause electrical charging, it becomes time-consuming, reducing productivity.